EP0573870A1 - Process for the manufacture of copolymers of ethylene and acrylic acid esters - Google Patents

Abstract

A process for the preparation of copolymers of ethylene with acrylic esters (acrylates) by copolymerisation of 100 parts by weight of ethylene with from 0.1 to 25.0 parts by weight of acrylic esters in a continuous, tubular polymerisation system at pressures of from 500 to 4,000 bar and temperatures of from 100 to 400 DEG C in the presence of polymerisation initiators which decompose to form free radicals, where the temperature regime for the reaction mixture comprising ethylene, acrylic esters, polymerisation initiators and optionally regulators after entry into the reaction zone is designed in such a way that the quotient dT/dt (increase in temperature per time unit) is kept constant at a value between 1.5 and 2.5 DEG C per second.

Description

The invention relates to a process for the preparation of copolymers of ethylene with acrylic acid esters by copolymerization of 100 parts by weight of ethylene with 0.1 to 25.0 parts by weight of acrylic acid esters in a continuously operated, tubular polymerization system at pressures of 500 to 4000 bar and temperatures of 100 to 400 ° C in the presence of free radical polymerization initiators.

The ethylene-acrylic acid ester copolymers formed in the above polymerization - hereinafter referred to as copolymers - are used, for example, as film material, cable sheathing or adhesives.

The process for the preparation of the copolymers takes place in tubular reactors which are described, for example, in "Ullmann's Encyclopedia of Industrial Chemistry", 4th edition, volume 19, pages 173-175.

The term ethylene-acrylic acid ester copolymers comprises copolymers with a copolymerized comonomer content of 0.1 to 25.0, preferably 0.5 to 20.0% by weight, which have a melt index of 0.1 to 25 g / 10 min. , determined according to ASTM-D 1238-65 T at a temperature of 190 ° C and a coating weight of 2.16 kg, and a density of 0.890 to 0.935 g / cm³, measured according to DIN 53 479.

All acrylic acid esters which can be copolymerized with ethylene at the stated temperatures and pressure ratios are suitable as acrylic acid esters. Such comonomers are, for example, esters of acrylic and methacrylic acid with C₁ to C₈, preferably C₁ to C₆ alkanols. Methyl, n-butyl and tert-butyl acrylate are particularly preferred. It is also possible to use mixtures of acrylic acid esters.

The ethylene is copolymerized with the acrylic acid esters in the presence of radical polymerization initiators. Free radical polymerization initiators are understood to mean those catalysts which are also used for the homopolymerization of ethylene under high pressure. Oxygen is suitable, for example, expediently in amounts of 2 to 100 mol ppm, based on the ethylene to be polymerized. Peroxides and other radical formers as well as mixtures of peroxides and mixtures of oxygen and peroxides and / or hydroperoxides are also suitable.

Examples of peroxides and hydroperoxides are: tert-butyl peroxy pivalate, di-tert-butyl peroxide, tert-butyl hydroperoxide, tert-butyl perbenzoate, p-menthane hydroperoxide and di-lauroyl peroxide. Free radical polymerization initiators are also to be understood as meaning compounds such as azoisobutyronitrile. Mixtures of oxygen and one or more peroxides can also be used. The copolymerization of ethylene with acrylic acid esters is preferably initiated by atmospheric oxygen.

Usually one works in the presence of polymerization regulators. With the help of the polymerization regulator it is possible to adjust the melt index of the ethylene copolymers formed. Examples of suitable regulators are hydrogen, ketones, aldehydes, alcohols, ethers or normal and branched hydrocarbons. Propane, propylene, methyl ethyl ketone or propionaldehyde are preferably used. The polymerization regulators are generally used in amounts of 0.2 to 5 mol%, based on the ethylene to be polymerized. In a particular embodiment of the process, there is no need for additional polymerization regulators if the comonomers used themselves serve as regulators or if substances which have regulator properties are formed during the copolymerization.

The copolymerization is generally carried out in the absence of a solvent. The small amounts of an inert solvent, such as benzene, mineral oil or other inert solvents, in which the peroxides used as polymerization initiators are dissolved, can be neglected compared to the other starting materials.

If oxygen is used as the polymerization initiator, any solvent can be omitted.

The preparation of the copolymers described above is known and is described in the patent literature.

In French Patent 1,596,991, the temperature in the reactor is chosen so high in the production of copolymers of ethylene with acrylic acid esters that the secondary and tertiary ester groups in the polymer are partially or completely pyrolyzed, free carboxylic acid groups being obtained in the copolymer with elimination of an alkene.

Other processes, such as those described in the English patents 1 010 847, 915 210 and 1 347 361 below, aim to increase the conversion of ethylene to polyethylene per throughput.

In the known processes, there are disturbances in the production process, especially with increasing operating time, which result from fluctuations in the reaction temperatures. Starting from the beginning of the reaction zone, the temperature fluctuations propagate in batches over the entire reaction zone. In extreme cases, very violent decomposition of the reaction mixture has been observed as a result of the instability.

The present invention was therefore based on the object of making the reaction of the copolymerization of ethylene with acrylic acid ester in tubular reactors so stable that the disorders mentioned were effectively eliminated and decompositions of the reaction mixture were ruled out.

The object is achieved in that the temperature control for the reaction mixture, consisting of ethylene, acrylic acid ester, polymerization initiators and optionally regulators, is designed after entering the reaction zone in such a way that the quotient dT / dt (temperature increase per unit of time) at a value between 1.5 to 2.5 ° C per second is kept constant.

A preferred choice of the quotient temperature increase per unit time is 1.6 to 2.3 ° C per second.

When using normal butyl acrylate, the quotient dT / dt should be kept in the range of 1.6 to 1.9 ° C per second. Exceeding the upper limit value must be avoided, since otherwise the reaction mixture may decompose within a short time.

In the copolymerization of ethylene with tertiary butyl acrylate, a somewhat higher temperature rise in the reaction mixture can be used as it passes through the first tubes of the reaction zone; in this case the quotient dT / dt can be set at values between 1.9 and 2.3 ° C per second. It is expedient to work with shorter reactors in the upper range of the stated values, since maximum conversions can be achieved here.

In the case of longer reactors, the volume available for converting the reactants is sufficient, so that dT / dt does not necessarily have to be operated at the upper limit of the quotient for the reactor to operate economically.

In any case, it is essential to avoid exceeding the upper limit, since decomposition of the reaction mixture has also been observed here.

The process works at pressures above 800 bar, preferably at a pressure of 1500 to 3000 bar, and temperatures between 150 and 350 ° C. As it enters the reaction zone, the reaction mixture immediately experiences an exact temperature control such that the quotient dT / dt - temperature rise per unit time for the reaction mixture as it passes through the first ten tubes of the reaction zone - is kept constant at a defined value according to the composition of the reactants and an upper limit of the quotient is not exceeded.

The temperature control in the first ten tubes of the reaction zone - this means a distance of 100 to 260 m, depending on the reactor type - is determined by the quantity and temperature of the cooling jacket of the reaction tubes passed cooling medium in interaction with the concentration of polymerization initiator and the gas temperature of the reaction mixture at the entrance to the reaction zone.

In a known manner, pressurized water in the temperature range from 160 to 200 ° C. is used as the cooling medium.

In a preferred embodiment of the method according to the invention, one works with a cooling device separate from the rest of the hot water cooling circuit for the section in which the quotient dT / dt must be kept constant.

The gas temperature of the reaction mixture at the entrance to the reaction zone is limited to a maximum of 175 ° C.

The process according to the invention surprisingly showed that a significantly more stable running behavior of the reactor was achieved.

If, on the other hand, the reaction mixture in the first tubes of the reaction zone is operated without a precisely defined temperature control, temperature fluctuations occur in the copolymerization of ethylene with normal butyl acrylate of up to 50 ° C, when using tertiary butyl acrylate from 10 to 20 ° C.

These relatively large temperature fluctuations can be explained by the reaction of the acrylic acid esters with themselves due to local overheating. The acrylic acid esters react many times faster than ethylene; This primarily forms poly-acrylates, which stick to the walls of the reaction tubes as stubborn deposits. This leads to a deterioration in the heat transfer, which increases the instability of the reaction or the temperature fluctuations. From a certain thickness of the wall covering, the heat transfer becomes so insufficient that the reaction mixture decomposes.

According to the method according to the invention, the disturbances mentioned do not occur; Surprisingly, there are only slight temperature fluctuations in the entire area of the reaction zone. Decomposition is required if the in the conditions set out in claims 1 to 3 did not come.

example 1

A mixture consisting of 1000 parts by weight of ethylene, 32 parts by weight of normal butyl acrylate and 15 mol ppm of oxygen, based on ethylene, and 0.3 part by weight of propylene was copolymerized as a polymerization regulator at a pressure of 2300 bar in a tubular reactor.

The reaction mixture was introduced into the reaction zone at a temperature of 171 ° C.

The temperature rise in the first ten tubes of the reaction zone was kept constant at a value of dT / dt = 1.8 ° C per second.

For this purpose, hot water with an inlet temperature of 172 ° C. was pumped in countercurrent through the cooling jackets of the reaction tubes; the flow rate of hot water was 7.4 m³ per hour for the cooling section in question.

The reaction mixture reached its maximum temperature of 270 ° C. in the second half of the reaction zone.

The temperatures across the entire reaction zone fluctuated only slightly. No decomposition occurred under the reaction conditions mentioned.

There were obtained 217 kg of a homogeneous ethylene-butyl acrylate copolymer which had good film properties.

The analyzes showed the following values: Melt index 1.36 ± 0.05 g / 10 min. density 0.9247 g / cm³ Normal butyl acrylate content 14.3 ± 0.1% by weight.

Example 2

A mixture of 1000 parts by weight of ethylene, 33 parts by weight of tert-butyl acrylate and 10 mol ppm of oxygen, based on ethylene, was reacted in a tubular reactor under a pressure of 2250 bar.

The reaction mixture was introduced into the reaction zone at a temperature of 167 ° C.

The temperature rise in the first ten tubes of the reaction zone was kept constant at a value of dT / dt = 2.25 ° C per second.

The cooling medium (hot water) that was pumped through the cooling jackets of the reaction tubes had a temperature at the inlet of 176 ° C. The flow rate of hot water was 8.3 m³ per hour for the cooling section in question. The reaction mixture reached its maximum temperature of 292 ° C. in the second half of the reaction zone.

The temperature over the entire reaction zone was subject to only slight fluctuations. Even after several weeks of operation, the response behavior was still extremely stable. No decomposition occurred under the reaction conditions mentioned.

220 kg of a homogeneous copolymer with good film properties were obtained.

The analyzes showed the following values: Melt index 7.4 ± 0.1 g / 10 min. density 0.9269 g / cm³ Content of tert-butyl acrylate 6.90 ± 0.05% by weight Free acrylic acid content 4.05 ± 0.05% by weight.

Comparative Example 1

As in Example 1, a mixture of 1000 parts by weight of ethylene, 32 parts by weight of normal butyl acrylate, 15 mol ppm of oxygen and 0.3 part by weight of propylene was used as a polymerization regulator under a pressure of 2300 bar Brought reaction. The polymerization conditions were the same as in Example 1 with one exception. In contrast to Example 1, the temperature control at the entrance to the reaction zone was not kept at a defined value, ie the quotient dT / dt for the reaction mixture when passing through the first ten tubes of the reactor was not kept constant.

In the second half of the reaction zone, the reaction mixture reached its maximum temperature, which fluctuated between 255 and 298 ° C., the position of the temperature maximum also changing and decomposition of the reaction mixture taking place.

250 kg of copolymer were obtained with the following analytical values: Melt index 1.30 - 1.36 g / 10 min. density 0.9243 g / cm³ Normal butyl acrylate content 14.2 - 14.5% by weight.

Comparative Example 2

As in Example 2, a mixture of 1000 parts by weight of ethylene, 33 parts by weight of tert-butyl acrylate and 10 mol ppm of oxygen, based on ethylene, was fed to the tubular reactor and polymerized under a pressure of 2250 bar.

With one exception, the reaction conditions were the same as those given in Example 2. In contrast to Example 2, the temperature rise of the reaction mixture after it entered the reaction zone was not deliberately kept at a constant value.

The reaction mixture reaches maximum temperatures of 280 to 296 ° C. in the second half of the reaction zone.

Fluctuations in the temperatures were observed in the entire area of the reaction zone, the position of the temperature maximum also changing and temporary decomposition of the reaction mixture occurring.

225 kg of a copolymer were obtained with the following analytical values: Melt index 7.45 - 7.60 g / 10 min. density 0.9273 g / cm³ Content of tert-butyl acrylate 6.85 - 7.10% by weight Free acrylic acid content 4.0-4.1% by weight.

Claims (3)

Process for the preparation of copolymers of ethylene with acrylic acid esters by copolymerization of 100 parts by weight of ethylene with 0.1 to 25.0 parts by weight of acrylic acid esters in a continuously operated, tubular polymerization system at pressures of 500 to 4000 bar and temperatures of 100 to 400 ° C in the presence of Radically disintegrating polymerization initiators, characterized in that the temperature control for the reaction mixture, consisting of ethylene, acrylic acid ester, polymerization initiators and optionally regulators, is designed after entering the reaction zone in such a way that the quotient dT / dt (temperature increase per unit of time) is between 1 , 5 and 2.5 ° C per second is kept constant. A method according to claim 1, characterized in that when using normal butyl acrylate the quotient dT / dt is set between 1.6 and 1.9 ° C per second and kept constant. A method according to claim 1, characterized in that when using tertiary butyl acrylate the quotient dT / dt is set between 1.9 and 2.3 ° C per second and kept constant.